X-ray tubes are used in various X-ray inspection devices such as an X-ray CT device which captures the condition of the internal structure of subjects such as a human body utilizing the transmission power of X-rays, an X-ray fluoroscope and nondestructive inspection analyzers for detecting defects inside a structure and the inside of a case (for example, baggage inspection systems).
Each of these X-ray inspection devices includes an X-ray tube which generates X-rays and an X-ray detection section provided with a scintillator (including an image intensifier tube) that detects X-rays transmitted through a subject.
The X-ray tube is generally provided with a pair cathode and anode arranged so as to face each other in a container made of a glass bulb, metal or ceramics. The cathode is constituted of, for example, tungsten filaments. Also, the anode is constituted of a target made of tungsten (W), molybdenum (Mo) or an alloy thereof. With regard to the principle of the function of this X-ray tube, electrons are emitted by heating the tungsten filaments of the cathode and accelerated by the voltage applied across the anode and cathode. These electrons then collide with a target which is the anode, as an electron beam having kinetic energy, with the result that X-rays are emitted in a predetermined direction from the target.
In recent years, X-ray CT devices, X-ray fluoroscopes and nondestructive examination analyzers are desired to have improved resolution associated with a highly precise X-ray image and to shorten the time required for obtaining a moving image and for inspection. In order to improve the resolution of X-ray CT devices, it is necessary that each individual X-ray scintillator to be used in the X-ray detection sections be small-sized and many of them be arranged in the same detection area. However, if these X-ray scintillators are small-sized, X-ray detection sensitivity to the same X-ray incident energy per unit area is reduced. This reduction in sensitivity can be compensated by more increasing the output of X-rays from the X-ray tube. Also, the reduction in the time required for obtaining a moving image and for inspection can be attained by more increasing the output of X-rays from the X-ray tube. From the reason mentioned above, there is a need for a high-output X-ray tube which can generate X-rays stronger than a conventional X-ray tube and therefore, high-output X-ray tubes are being developed and put to practical use.
Generally, it is necessary to increase the kinetic energy of electrons which collide with a target to increase the output of X-rays of the X-ray tube. However, a part of the kinetic energy of electrons acts as thermal energy to raise the temperature of the part where electrons collide with the target, with the result that melting of the target itself and the rise in temperature cause deteriorations in the metal phase of the target.
For this, many high-output X-ray tubes adopt a structure in which an axially symmetric rotating body (for example, a disk shape) is used as the target and the rotating body is rotated at a speed as high as 2000 rpm to 10000 rpm with respect to an electron beam to thereby always vary the focal plane of the target which receives electron beam radiation, thereby preventing a local rise in temperature. X-ray tubes having such a target are called rotating anode (target) X-ray tubes.
There are following methods used to develop such a rotating anode X-ray tube having a higher output: (1) a method in which the rotating speed of the target is more increased to thereby improve cooling efficiency and further, the kinetic energy of the electron beam which collides with the target is increased and (2) a method in which the target is large-sized to thereby widen the area with which the electron beam collides.
Emission from the X-ray tubes of X-ray CT devices and X-ray inspection devices is not continuous but these X-ray tubes each have idle time (time during which no energy is input into the target) during which no X-ray is emitted till the next inspection since one CT inspection or one nondestructive inspection is finished. For this, the heat capacity of the whole target is designed to have a large value, thereby making it possible to drop the maximum temperature of the target when X-rays are applied and also to raise the average temperature of the target and it is therefore possible to deal with high output of the X-ray tube. When the heat capacity of the whole target is increased, the mass of the target is desirably as small as possible because the target is a rotating body. The part with which the electron beam collides on the targets needs W, Mo or an alloy thereof as mentioned above. However, when W, Mo or an alloy thereof which has a large density and a small specific heat is only used to constitute the target, their mass is excessively large and therefore, this is undesirable. Therefore, in order to provide a large heat capacity and to limit an increase in the target weight, it is desirable to use carbon which is a material having no problem concerning mechanical strength under high temperature and a large specific heat by joining the carbon with W, Mo or an alloy thereof.
Also, though a method is considered in which the size of the target of W, Mo or an alloy thereof is more increased to enlarge the area of radiation as the method of attaining an X-ray tube target of a high-output X-ray tube, this method requires a structure taking the aforementioned rotating body stiffness standing to high rotation into consideration and large increases in the weight and dimension of the X-ray tube including a bearing. Moreover, when the target is used for a CT device showing a trend toward high-speed scanning, it is necessary to rotate the whole X-ray tube of the CT device at the CT scanning speed, and therefore the formation of the structure enough to stand to large centrifugal force is accompanied by a large difficulty.
A light-weight target can be attained if an X-ray tube formed by joining W, Mo with carbon as mentioned above is used. Such an X-ray tube target is reported in Patent Documents 1 and 2. Patent Document 1 discloses the use of V (vanadium) as a solder. In Patent Document 2, a method in which a metal base material is coated with a solder in advance by a sputtering method and then joined with a material is used.